Wireless or radio frequency (RF) communication systems are an integral component of the ongoing technology revolution and are evolving at an exponential rate. Many wireless communication systems are configured as “cellular” systems, in which the geographic area to be covered by the cellular system is divided into a plurality of “cells”. Mobile communication devices or stations (e.g., wireless telephones, pagers, personal communications devices and the like) in the coverage area of a cell communicate with a fixed base station or transmitter within the cell. Low power base stations are utilized, so that frequencies used in one cell can be re-used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a cell served by a base station.
The communication format used in most wireless communications systems is a high-frequency carrier waveform modulated by low frequency or “baseband” audio or data signals. Mobile stations (wireless handsets, for example) within a wireless communication system typically have a transmitter that “modulates” baseband signals (e.g., speech detected by the handset microphone) onto the carrier waveform. Amplitude modulation (AM) and frequency modulation (FM) techniques, for example, are well known to those of ordinary skill in the art. Mobile stations also typically have a receiver that “demodulates” the carrier waveform to extract the baseband signal.
The carrier waveform that is modulated is usually a high frequency, periodic waveform generated by an oscillator. The frequency of the oscillator should be adjustable since the transmitter is often required to transmit on many different frequency channels within a transmission band. In a GSM wireless network, for example, the transmission band is 880-915 MHz and is divided into 200 kHz channels. Thus, the oscillator frequency must be varied in precise steps of 200 kHz. Voltage-controlled oscillators (VCOs) are well suited for such applications since their output frequency is easily adjusted by manipulating a control voltage.
Many transmitters use phase-locked loops (PLLs) to generate the desired range of frequencies. The design and operation of PLLs is well known to those of ordinary skill in the art. A basic PLL 10 is illustrated in FIG. 1. It includes a VCO 16 that outputs a signal having a frequency fTx within a defined transmission frequency band. PLL 10 also uses a reference or clock signal having a frequency fREF equal to the required step size or frequency resolution (e.g., the channel bandwidth) of the PLL. Each frequency channel (e.g., 900 MHz, 900.2 MHz, 900.4 MHz) is an exact integer multiple of the reference frequency (e.g., 0.2 MHz). VCO 16 locks to the reference signal and tracks any modulation contained in the reference signal (to the extent that it is passed through the loop filter).
Frequency divider 18 divides the frequency of the VCO output signal by an integer N to yield a signal having the same frequency as the reference frequency,       f    REF    =                    f        Tx            N        .  The divided frequency and reference frequency signals are input to phase detector 12, which compares the phases of the two signals and outputs a control voltage to control the frequency of the VCO. The output from phase detector 12 is usually passed through a charge pump/loop filter (14) before being supplied to VCO 16. Hence, the output frequency of VCO 16 can be programmed in discrete steps by changing the value of divider N. Passing the reference frequency through a reference divider (not shown), thereby making the step size programmable, can provide additional flexibility.
Some wireless transmitters use a translation loop architecture that is very similar in concept and operation to a PLL architecture. An exemplary translation loop transmitter 200 is illustrated in FIG. 6. Transmitter 200 will be discussed briefly for now and in more detail in the description to follow. In a translation loop transmitter, modulation is typically performed by a quadrature mixer (202) that modulates baseband audio and/or data signals onto a carrier wave at an intermediate frequency (IF) generated by a local oscillator (LO1). The IF frequency is also used as the reference input to a phase detector (204). The frequency of the modulated VCO (210) output signal is down-converted to the IF frequency for phase comparison by mixing the VCO signal (mixer 212) with a signal generated by a second local oscillator (LO2).
Translation loop transmitters have several drawbacks. Frequency mixers generate various cross products of the local oscillator signals and their harmonics. Spurious mixing products can also be created through leakage of local oscillator signals. In FIG. 6, for example, the signal from oscillator LO1 may leak to oscillator LO2, and vice-versa, to generate mixing products. Though filters are employed to remove these mixing products, low frequency products (“zero crossing” spurs) may not be attenuated by the filters and may cause corruption and spurious modulation of the VCO transmit signal. Additionally, translation loop transmitters generally require use of a quadrature mixer or modulator, which increases the required circuitry and decreases the cost efficiency of the transmitter.